Hydrogen sulfide gas has a number of undesirable properties. At low concentrations (i.e., less than 10 parts per million), the gas has a strong odor of “rotten eggs”. Paradoxically, at concentrations greater than 100 parts per million (ppm), the gas can no longer be smelled, but it will induce dizziness. Concentrations of greater than about 500 ppm can be fatal to humans. Hydrogen sulfide can also be oxidized to sulfur dioxide, a chemical that contributes to acid rain.
Hydrogen sulfide (H2S) gas can be released by a variety of sources. Biologically-generated hydrogen sulfide is generally the product of anaerobic digestion of organic matter, and can be released by sewage treatment facilities, solid waste landfill, paper mill waste, cattle feed lots, poultry farms, and other industries employing anaerobic digestion for processing. H2S is also released by oil drilling operations, oil refinery operations, and operations utilizing geothermal brines, such as for power generation.
Biological technologies are frequently used for H2S abatement. Sulfur bacteria (i.e., bacteria that are able to metabolize sulfur compounds) can be utilized in biofilter or bioscrubber reactors to oxidize H2S to sulfates. Biofilter and bioscrubber plants are normally designed to produce elemental sulfur. In this process, H2S gas is passed over the bioreactor bed under aerobic conditions, leading to reduction of the H2S to elemental sulfur that is deposited intracellularly and extracellularly as a solid in clumps or granules. This elemental sulfur is mechanically removed periodically from the bioreactor bed. The deposited sulfur, once removed, may be disposed of or utilized in industrial and/or agricultural processes.
The invention herein relates to processing gas streams to remove hydrogen sulfide using an alkaline solution containing Thiobaccilus bacteria (Thiopaq). The alkaline solution contains solid particles of elemental sulfur in the 1 to 20 micron size range. The presence of the sulfur particles is essential to proper operation of the biological H2S removal process. An unfortunate side effect of most of these biological sulfur removal processes is formation of foam that can be detrimental to the proper operation of the process.
In treating gas streams, especially natural gas streams, it is important that H2S not be carried to the downstream equipment or pipelines because of possible fouling or corrosion. The formation of foam in the processing equipment can be caused by excessive sulfur solids concentration, flashing of dissolved gas during pressure reduction, counter current gas/solution flow, agitation, shear through spray nozzles and pumps, presence of surfactants and other causes.
Conventional separation devices are not effective in handling this type of foam. The invention herein applies high G forces to the foam containing gas stream using a cyclonic separation device. It has been determined that high G forces can be used to cause the foam to collapse and allow the sulfur and solution to properly separate from exiting gas streams. Laboratory pilot testing has confirmed that at a certain level of G forces the most persistent foams could be broken and the gas stream cleanly separated from the solids and liquids in the foam. This level of G forces was not intuitive, and is an order of magnitude more G's than would typically be expected.
For additional information relating to biological gas sweetening systems and processes, reference may be had to the following previously issued United States patents.
PatentNumberInventorTitle4,838,434Miller et al.Air Sparged Hydrocyclone FlotationApparatus and Methods For SeparatingParticles From A Particulate Suspension4,997,549AtwoodAir-Sparged Hydrocyclone Separator5,112,375BrownRadial Vane Demisting System In ASeparator For Removing EntrainedDroplets From A Gas Stream5,192,423Duczmal et al.Apparatus and Method For SeparationOf Wet Particles5,522,510Luttrell et al.Apparatus For Improved Ash and SulfurRejection6,162,284Mitchell et al.Separator For Gases, Liquids and SolidsFrom A Well6,773,492WestSystem Employing A Vortex Tube ForSeparating An Entrained LiquidComponent From A Gas Stream6,576,029WestSystem For Separating An EntrainedLiquid Component From A Gas Stream